The invention relates to a method for identification and partial-pressure determination of two gases in an unknown gas mixture comprising n possible gases, by measurement of the infrared optical radiation emitted by a radiation source and transmitted through a cuvette holding the unknown gas mixture, which radiation passes through m filters with different transmission wavelength ranges; n is a number greater than 2, and m is a number less than n.
The gas absorbs infrared optical radiation in a very specific wavelength range, and the transmission of the unabsorbed infrared optical radiation in this wavelength range is characteristic for the gas and its partial pressure.
A method for identifying and determining the concentration of at least one gas in an unknown gas mixture comprising n possible gases by measuring the infrared optical radiation transmitted through the gas mixture is described in U.S. Pat. No. 5,731,581. This uses light radiation and more than n different wavelength ranges, which strikes filters that are each permeable to these wavelength ranges. From the measured values, the at least one gas present in the unknown gas mixture is identified and its concentration determined, by multivariant statistical methods. The concentration of a gas is obtained from the quotient of the partial pressure of the gas and the total pressure. If the total pressure is known, then from the partial pressure it is always possible to learn the concentration of a gas, and vice versa.
A disadvantage of the known method is that for identifying and determining the concentration of at least one gas in an unknown gas mixture, a large number of filters and respective downstream detectors are required, specifically at least as many filters and detectors as there are possible gases in the gas mixture.
The object of the present invention is to disclose a method for identification and partial-pressure determination of two gases in an unknown gas mixture that can be performed by structurally simple means.
According to the invention, this object is attained by a method having the steps of claim 1.
The method for identification and partial-pressure determination of two gases in an unknown gas mixture comprising n possible gases by measuring the infrared optical radiation emitted by a radiation source and transmitted through a cuvette holding the unknown gas mixture, which radiation passes through m filters with different transmission wavelength ranges, in which n is a number greater than 2, and m is a number less than n, comprises the following steps.
First, either a single time or at relatively long time intervals, such as several weeks, the evaluation and control unit performs a calibration, before the actual identification and partial-pressure determination in the unknown gas mixture is performed. For each individual one of the n gases that possibly occurs in the unknown gas mixture, and for each of the m filters, which are each permeable to a special wavelength range, the exit intensity of the infrared optical radiation transmitted through the gas, once it has passed through the filter, is determined as a function of the partial pressure of the gas. This dependency can be described by means of a calibration curve. A total of nxc3x97m different calibration curves are obtained, for each of the n gases and each of the m filters.
This is followed by the identification and partial-pressure determination of two gases in an unknown gas mixture. This concept will always be understood hereinafter to include the identification and partial-pressure determination of one gas instead of two gases as well, in the event that only this one gas occurs in the unknown gas mixture. That case is a simplified special case, which will no longer be mentioned separately below but instead will be included in the concept of the identification and partial-pressure determination of two gases. For each of the m filters, the exit intensity of the infrared optical radiation transmitted through the unknown gas mixture, once it has passed through the filter, is measured by a detector downstream of the filter. The result is m values.
On the basis of an evaluation of the previously created nxc3x97m calibration curves, the evaluation and control unit for each of the possible gas mixtures, each of which comprise two of the n gases and will hereinafter be called a mixture pair, determines the particular pair of partial pressures whose associated m exit intensities best correspond to the m exit intensities measured in the unknown gas mixture, in accordance with a specified distance measure. A distance measure for determining which exit intensities, belonging to a pair of partial pressures, best correspond to the exit intensities measured in the unknown gas mixture is for example the sum of the distances between the exit intensities of one mixture pair, for a particular pair of partial pressures, and the measured exit intensities of the gas mixture, or the sum of the squares of the distances of the exit intensities of a mixture pair for a particular pair of partial pressures and the measured exit intensities of the gas mixture.
How the evaluation is done will be explained hereinafter, taking one mixture pair as an example.
Once, for each of the possible mixture pairs, the particular pair of partial pressures has been determined in which the associated exit intensities best correspond to the measured exit intensities of the unknown gas mixture in accordance with the specified distance measure, then from among the particular pairs of partial pressures, the partial pressure whose associated exit intensities best correspond to the measured exit intensities of the unknown gas mixture is ascertained. The two gases of the mixture pair that belong to this pair of partial pressures are identified as the two gases of the unknown gas mixture. Their ascertained partial pressures can optionally be used to determine the concentration, by means of division by the total pressure.
For the method, an infrared optical radiation gas measuring instrument is used that includes an infrared optical radiation source, a cuvette for the gas specimen to be measured, a plurality of optical filters with different transmission wavelength ranges, and at least one downstream detector for measuring the exit intensity of the infrared optical radiation, as well as an evaluation and control unit.
In a preferred embodiment, the method is employed for identification and partial-pressure determination of two gases in an unknown gas mixture comprising five possible gases. These can in particular be the five anesthesia gases, that is, halothane, enflurane, isoflurane, sevoflurane, and desflurane.
In a further preferred embodiment, three filters with different transmission wavelength ranges are used.
An embodiment in which four filters with different transmission wavelength ranges are used is equally possible.
The embodiments with three and four filters used are shown as examples in the drawings, and the embodiment with three filters will be described in detail below.